Although ethylene diamine tetracetate (EDTA) has been known in the prior art as an anticoagulant for use in the collection of blood samples, its normal use has heretofore been limited to the partially evacuated tubes used for collecting relatively large blood samples, such as 2 ml or more for example, from the hypodermic puncture of a patient's vein.
It is often not practical nor advisable to perform a venipuncture in certain medical cases, such for example as pediatrics, geriatrics, severe burns, and the like. In such cases, a "micro" sample of blood is obtained from a superficial skin puncture of the fingertip, heel, or earlobe. A typical "micro" sample volume of 250 .mu.l is obtained using this procedure. The specimen is collected directly from a blood droplet on the skin surface into a glass micro-pipette. If an anticoagulant is not mixed into the blood sample, it will clot within a few minutes.
Prior art blood microsample collection tubes have been coated on the inner walls with a small amount of the anticoagulant heparin, typically in amounts of 25 .mu.g in a 250 .mu.l tube. However, heparinized blood has staining characteristics which make it unsuitable for microscopic examination. Ethylene diamine tetracetate is the anticoagulant of choice for tests involving microscopic examination since it preserves cellular morphology fairly well and does not introduce any staining artifacts. However, ethylene diamine tetracetate has a relatively high minimum effective concentration, typically 250 .mu.g in a 250 .mu.l sample tube. This factor alone has seriously hindered previous attempts to produce ethylene diamine tetracetate coated microsample tubes although other serious problems included poor adhesion of the dry ethylene diamine tetracetate to the glass tube walls, as well as poor mixing of the dry EDTA with the blood sample, resulting in the formation of small clots.